Device for calibration of magnetic sensors in three dimensions

ABSTRACT

The invention refers to a magnetic calibration device comprising a mounting means designed to support at least one magnetic sensor card being detachably attached and comprising at least one magnetic sensor, in particular in form of a Hall sensor, to be calibrated and connected to a first analog electronic circuit with at least one current source as well as at least one first analog to digital converter and at least one coil card being detachably attached and comprising three coils arranged substantially orthogonal to each other and connected to a second analog electronic circuit with at least one second analog to digital converter; at least one connection means, in particular in form of a cable or a wireless link, for applying at least one supply voltage to the first and second analog electronic circuits, respectively, and for guiding digital signals from the first and second analog to digital converter, respectively, to at least one processing unit; a magnet for generating a substantially homogeneous and constant calibration magnetic field; and a rotator for rotating said cards in said calibration magnetic field around two substantially orthogonal axes.

The present invention refers to a magnetic calibration device, inparticular for calibrating Hall sensors in three dimensions. It iscommon to use Hall sensors to determine the strength of a magnetic fieldpresent in medical, physical or technical setups or systems. Forexample, EP 0 947 846 A2 discloses a three dimensional (3D) magneticfield sensor for measuring the three components of a magnetic fieldcomprising a Hall sensor and an electronic circuit The Hall sensorcomprises an active area of a first conductivity type in connection withvoltage and current contacts. Four voltage contacts are present whichare, in turn, connected to input terminals of the electronic circuit. Bymeans of summation and subtraction, respectively, of selected electricalpotentials of the voltage contacts, the electronic circuit derives threesignals which are proportional to the three components of the magneticfield. Additionally, the magnetic field sensor has the advantage that itmeasures all three components of the magnetic field at a common volumeor point due to the fact that the common active area has a size oftypically only 100 μm*100μm up to 300 μm*300 μm. However, since the Hallsensor exhibits a high cross sensitivity between the components of themagnetic field there is a need of a reliable, precise calibration,especially in case one is interested in a high resolution even in highmagnetic fields.

A conventional two axes calibration device with a rotator would requirehigh precision position or angle encoders to achieve high resolution.Besides their inexpedient sizes encoders of that kind are veryexpensive. Moreover the rotator of the calibration device must bedesigned to have a play less than 104 rad resulting in a cost increasingproduction.

It is therefore the object of the present invention to provide amagnetic calibration device overcoming the drawbacks of the prior art.In particular, it is the object of the present invention to provide asimple calibration device for calibrating a magnetic field sensor, whichis capable of calibrating a magnetic filed sensor with a high precision,i.e. a high resolution. It is another object of the present invention toprovide a calibration device which measures all three components of amagnetic field independent of its orientation relative to the magneticfield.

This object is achieved by a magnetic calibration device comprising amounting means de-signed to support at least one magnetic sensor cardbeing detachably attached and comprising at least one magnetic sensor,in particular in form of a Hall sensor, to be calibrated and connectedto a first analog electronic circuit with at least one current source aswell as at least one first analog to digital converter and at least onecoil card being detachably attached and comprising three coils arrangedsubstantially orthogonal to each other and connected to a second analogelectronic circuit with at least one second analog to digital converter;at least one connection means, in particular in form of a cable or awireless link, for applying at least one supply voltage V_(S) to thefirst and second analog electronic circuits, respectively, and forguiding digital signals from the first and second analog to digitalconverter, respectively, to at least one processing unit; a magnet forgenerating a substantially homogeneous and constant calibration magneticfield; and a rotator for rotating said cards in said calibrationmagnetic field around two substantially orthogonal axes.

In one embodiment of the invention the rotator comprises two conicalgears, such as tooth gears or roll gears, two substantiallyconcentrically arranged shafts and at least one driving unit for theshafts, in particular in form of a stepper engine controller connectedto two engines and/or connected to the shafts via worm wheels.

It is preferred that the at least one driving unit is arranged outsidethe calibration magnetic field.

According to the invention the speed and direction of rotation of thetwo shafts can be controlled by the at least one driving unit to coverthe full ranged of polar and azimuthal angles of the unit sphere by acontinuous movement, preferably including a time reversed rotation forcompensating induction effects in the at least one magnetic sensor.

In addition, in one embodiment of the invention the magnetic calibrationdevice is at least partly formed by vibration damping and non-conductingmaterial, preferably at least one of the shafts comprises heavy,non-conducting material and/or is arranged in slide bearings.

With the invention it is proposed that the amount of turns of the innershaft differs from the amount of turns of the outer shaft by one turnwithin one calibration cycle, the cable being preferably turned onlyonce within one calibration cycle.

It is preferred that the at least one processing unit is arrangedoutside the calibration magnetic field and stationary.

In yet another aspect of the invention several magnetic sensor cardsand/or at least one magnetic sensor card and the coil card are stackablenext to each other, preferably closely spaced apart.

In one embodiment at least one dowel pin, screw, plug, clamp and/or clipto precisely and reproducibly position at least one magnetic sensor cardand/or coil card is provided.

According to the invention the space occupied by the three coils on thecoil card can amount up to around 10×10×10 mm³ for magnetic fields inthe range of about 0.1 to 2.0 Tesla.

In still another embodiment of the invention the coils are wound from 20μm wire into the form of cylinders with a diameter and height each of upto around 5 mm, for magnetic fields in the range of about 0.1 to 2.0Tesla and/or are each surrounded by a grounded electrostatic shield.

In another aspect of the invention one magnetic sensor card carries one3-dimensional or one 2-dimensional and one 1-dimensional or three1-dimensional Hall sensor (s).

It is proposed with the invention that the first analog electroniccircuit comprises at least one low pass filter, multiplexer and/ordelta-sigma modulator.

It is also proposed with the invention that the second analog electroniccircuit comprises at least one low pass filter, multiplexer and/ordelta-sigma modulator

It is preferred that the first and/or second electronic circuit, inparticular the at least one low pass filter, is arranged in the regionof the center of rotation of the cards.

A further embodiment is characterized by the process unit comprisingmeans for integrating the digital coil signals to obtain the componentsof the calibration magnetic field in angular coordinates relative to thecoils; means for decomposing the digital magnetic sensor output voltagesinto spherical harmonics on the basis of the obtained angularcoordinates; and means for storing the obtained coefficients of thespherical harmonics as a function of the calibration magnetic field toobtain a calibration table.

Optionally in one embodiment of the invention there is at least onemagnetometer, in particular at least one NMR magnetometer and/or fixedHall sensor, arranged within the calibration magnetic field andconnected to the processing unit to provide the absolute value of thecalibration magnetic field to the processing unit.

The processing unit preferably comprises means for performing atransformation of the obtained angular coordinates for aligning thereference frame with the symmetry axes of the at least one magneticsensor.

In addition, one embodiment of the invention is characterized by atleast one temperature sensing element, comprised by a thermistor or theHall sensor and connected to the processing unit in order to supply thetemperature within the calibration magnetic field, in particular of theHall sensor, to the processing unit.

With the invention it is preferred that the processing unit comprisesmeans for obtaining the temperature of the Hall sensor by decomposingthe Hall input voltages depending on the magnitude and direction of thecalibration magnetic field and the temperature into spherical harmonicson the basis of the decomposition of the Hall output voltages.

In still another embodiment of the invention a thermal insulating boxhousing the mounting means with the magnetic sensor and coil cards isprovided, said thermal insulating box preferably connected to a controlcircuit for controlling the temperature within the box, measured inparticular by the temperature sensing element.

In one ether aspect of the invention the control circuit, preferablycomprised by the processing unit, comprises a Peltier element forcooling and/or heating, at least one ventilator, preferably driven by anengine outside the calibration magnetic field, and a controller.

With the invention it is also proposed that the current source of theHall sensor is either a constant current source or a precise voltagesource with an internal resistance substantially equal to the inputresistance of the Hall sensor.

Optionally in one embodiment of the invention at least two magneticsensor cards are supported by tie mounting means, one card carrying atleast one calibrated Hall sensor and each remaining card carrying atleast one Hall sensor to be calibrated by comparison with the at leastone calibrated Hall sensor.

Finally in one embodiment of the invention the processing unit iscomprised by a microprocessor and/or personal computer.

Accordingly, with the invention a magnetic calibration device isprovided, which rotates continuously around two orthogonal axes threesubstantially orthogonal coils and for example one 3-dimensional Hallsensor to be calibrated in a constant homogeneous magnetic field. Thefull range of polar and azimuthal angles is covered by a respectiverotation. The parts of the device to be rotated are made compact to fitin between pole pieces of a special magnet. The magnetic field has to beconstant and homogeneous at least across the space covered by the coilsand the Hall sensors during rotation. Such a magnetic field can be foundin a special magnet with optimized coil and pole dimensions or at thecenter of a large magnet. The larger the homogeneous region of themagnet, the more Hall sensors can be calibrated at the same time.

The Hall sensors to be calibrated are precision fit on the magneticcalibration device of the invention by means of dowel pins, screws orclamps. The orientation of the coils and the Hall sensors is derivablefrom the integrated coil voltages.

With the device of the invention a calibration of Hall sensors againstat least one of the calibrated Hall sensors is possible, minimizing thecalibration time.

A special rotator is used in the magnetic calibration device of theinvention for a smooth rotation, necessary to avoid orientation errors.Every abrupt movement of a rotator would give rise to a peak in the coilvoltages and an increased error in the integration thereof necessary toobtain the components of the calibration magnetic field. The rotator isdriven by two concentric axes, the outer one drives a flange whichcarries a mounting means for coil cards and Hall sensor cards. Themounting means is driven via two conical tooth gears by both concentricaxes. The conical tooth gears can be replaced by conical roll gearswithout teeth. The movement becomes in this case smoother, but becauseof slip the positioning is less precise. By choosing the proper speedand direction of rotation of the axes the full range of polar andazimuthal angles is covered by a continuous movement. The speed islimited by the vibration level. The inner axes should make one turn moreor one turn less than the number of turns of the outer axes to end atthe same position as started from. This means that even after acalibration of many turns of the axes cables make only one effectiveturn. Making a calibration with time reversed rotation can determine theeffect of induction in the Hall circuit.

Heavy, non-conducting material is used to reduce vibrations and to avoidEddy currents. Mechanical damping is achieved by using silicon grease inslide bearings of the shafts and gears.

The mounting means is fixed by two centering screws with hand grip tothe flange on the outer axis. This makes it easy to remove the mountingmeans from the rest of the device to change the Hall sensor cards to becalibrated outside the magnet.

The magnetic calibration device of the invention is contained in athermal insulation box, the temperature of which is regulated by aPeltier element and controller. The controller comprises a thermistorwhich is placed close to the rotating mounting means. The external partof the Peltier element is ventilated by a fan outside the magneticfield. Inside the insulation box a ventilator stirs the air in order toget uniform temperature. The ventilators are driven by engines outsidethe magnetic field.

The electronics of a magnetic calibration device of the inventioncomprises for each coil a low-pass filter and an analog to digitalconverter, for example a delta sigma modulator. The output voltages ofthe Hall sensor to be calibrated are also converted into digital signalsby a delta sigma modulator. The pass band of the low-pass filters andsample rates of the delta sigma modulators are optimized to reduceelectronic and mechanical noise of the system, in particular to giveacceptable integration errors. All field sensitive components on thecoil cards and Hall sensor cards are arranged as close as possible tothe rotation center of said cards. All leads are kept short and withsmall loop surface against parasitic induction.

The analog to digital converters need a voltage reference. Since it isdifficult to design a voltage reference which is completely fieldin-sensitive, it is not placed on the rotating platform but stationaryand as close as possible to the analog to digital converter, connectedby a tight twisted pair cable kept in a flexible cable guide. Thisflexible cable guide contains also twisted pair leads of supply voltagesand digital signals of the analog to digital converters. The digitallines of the coil cards and Hall sensor cards can be shared. The supplyvoltage leads can be replaced by a system of collectors, and the digitallines can be replaced by wireless links. In this case a flexible cablecan be omitted and vibrations are further reduced.

The coils are formed as cylinders with typical diameters and heights of5 mm, filled with windings of wire with small possible diameters (0.02mm) for magnetic fields of around 0.1 to 2.0 Tesla They are surroundedby grounded electrostatic shields.

The stability of the magnetic field is monitored by a fixedNMR-magnetometer as close as possible to the magnet center and byseveral fixed Hall sensors.

The Hall sensor card is supplied with a small Hall current, so it willdissipate little heat and follow ambient temperature. No thermostat forthe Hall sensor is needed, whereas a temperature sensor close to itmakes it possible to calibrate at fixed temperatures. Alternatively, thetemperature of the Hall sensor is determined by measuring its inputvoltage as a function of both the magnetic field and temperature.

As a current source for the Hall sensor a precise voltage source with aninternal resistance substantially equal to the input resistance of theHall sensor can be used. In this case the heat produced in the Hallsensor would change considerably less due to internal resistance changesof the Hall sensor as a function of magnetic field, resulting in a morestable temperature. With a more stable temperature an extra thermistoris not needed, in particular when processing the Hall output and inputvoltages to obtain information on the magnetic field and temperaturevalues.

No absolute reference voltage for the whole sensor analog digitalconverter is used, as the analog digital converter reference voltage andthe Hall current are made linearly dependent from the supply voltage.Therefore, small changes in the supply voltages do not influence thesensitivity of the Hall sensor. A circuit to control the analog todigital converter sensitivity is foreseen.

A three dimensional Hall sensor can be build from three one dimensionalsensors, one two dimensional sensor plus one dimensional sensor or onethree dimensional sensor.

The different coil cards and Hall sensor cards are addressable via aserial data line, many serial data lines being put on same bus.

An algorithm is used by the magnetic calibration device of the inventionwhich is based on a decomposition of Hall voltages as a function ofazimuthal and polar angles into spherical harmonics. Due to theorthogonality of the spherical harmonics this decomposition is unique.After the decomposition a transformation is made to align the referenceframe of the spherical harmonics with the symmetry axes of the Hallsensors which makes it easy to compare different Hall sensors. Themaximum order of spherical harmonics one can extract is about equal tothe number of turns of the main axes of the rotator.

By interpolation in a table of coefficients of spherical harmonicsmeasured at different values of magnetic field and temperature it ispossible to reconstruct the components of a magnetic field with highprecision from three measured Hall voltages and the temperature. Alsothe input Hall voltage can be decomposed as a function of magnetic fieldand temperature in order to reconstruct the magnetic field and thetemperature, omitting a separate temperature sensor. The advantage of aone chip 3-dimensional Hall sensor in this case is that there is onlyone Hall input voltage.

The invention, together with further objects and advantages, may be bestunderstood, by example, with reference to the following description ofone embodiment taken together with the accompanying schematic drawings,in which

FIG. 1 is a cross sectional view of a magnetic calibration deviceaccording to the invention;

FIG. 2 is a perspective side view of the magnetic calibration device ofFIG. 1;

FIG. 3 is a block diagram of the electronic circuit on a Hall sensorcard used in the magnetic calibration device of FIGS. I and 2;

FIG. 4 is a side view of an exemplary coverage of the unit sphere duringcalibration for 4 turns of the outer axis;

FIG. 5 is a top view of the coverage of FIG. 4; and

FIG. 6 is a perspective view of the coverage of FIG. 4.

In FIG. 1 and FIG. 2 a magnetic calibration device I of the invention isillustrated comprising a rotator 2 composed of a support plate 3 whichis fixed via mounting screws 4 on a flange 5. This flange 5 is securedon an outer shaft 6 which is driven by a first worm wheel 7. An innershaft 8 lying concentrically within the outer shaft 6 is driven by asecond worm wheel 9. The second and first worm wheels 7, 9 are in turndriven by two engines (not shown). The whole shaft assembly is supportedby slide bearings 10 and 11 supporting the outer shaft 6 and inner shaft8, respectively. The slide bearings 11 for the outer shaft 6 areencapsulated in two holders 12.

At the support plate 3 facing end of the inner shaft 8 a first conicalgear 1I is arranged which meshes with a second conical gear 14perpendicular to gear 13. The second conical gear 14 is secured torotate in the support plate 3 with a mounting plate 15 screwed to it Fordamping mechanical vibrations the second conical gear 14 as well as theslide bearings 10, 11 are provided with silicon grease. In order toprevent any leakage of grease, the second conical gear 14 and themounting plate 15 are sealed with O-rings 16 against to the supportplate 3. The conical gears 13, 14 can either be conical tooth gears orconical roll gears which do not have any teeth. The main difference isthat the conical roll gear will provide a somewhat smoother movement ofthe rotator whereas the positioning is less precise because of slip. Onthe second conical gear 14 a coil card 17 is mounted, said coil card 17carrying substantially all, in particular analog, electronics includingthree orthogonal coils 18. Three holes 19 are distributed over the coilcard 17 to receive screws or dowel pins (not shown) for supporting orrather precision positioning the card as shown in FIG. 1 or 2. One ormore Hall sensor cards are mounted in the same way. The coil card 17 isrotatable together with the Hall sensor card(s) around two orthogonalaxes via the concentric shaft assembly 6 to 9 and the conical gears 13,14 in order to cover the full range of polar and azimuthal angles asshown in the FIGS. 4 to 6.

A Hall sensor card 17 a as shown in FIG. 3 comprises an electroniccircuit to which a supply voltage V_(S) is applied. By the help ofvoltage deviders in form of resistors 24 to 27 a reference voltageV_(Ref). for the analog to digital converter 20 a and a regulationvoltage V_(Reg.) of a current source 22 for a Hall sensor 23 isobtained. This ensures that the reference voltage V_(Ref.) and the Hallcurrent I_(Hall) are both made linearly dependent from the supplyvoltage V_(S). Therefore small changes in the supply voltages Vs do notinfluence the sensitivity of the Hall sensor 23. The Hall sensor 23 isconnected to the low pass filter and analog to digital converter 20 a.

The whole calibration device I is manufactured out of heavynon-conducting material(s) to reduce or inhibit both vibrations and eddycurrents during movement For example, the shaft assembly 6 to 9 is madeout of bakelite and the conical gears 13, 14 are made out of anacetalhomopolymer named delrin. Suitable are other phenolic resins orpolyacetals or synthetics in general too which have the above mentioneddesired characteristic features.

For calibration purposes the device 1 is arranged in a constant andhomogeneous field between the poles of a magnet (not shown). The twoengines driving the worm wheels 7, 9 are placed in a remote locationoutside any magnetic field of relevant strength. Driving the outer shaft6 in a certain direction by means of one particular engine, indicatedfor example in figure 2 by the arrow A, the flange 5 and, thus, the coilcard 17 and Hall sensor card 1 7a are rotating in the same manner. Arotation of the inner shaft 8 via the second engine, indicated in FIG. 2by the arrow B, sets the coil card 17 and Hall sensor card 17 a rotatingin a direction shown by the arrow C in FIG. 2. In particular, thecoverage of a unit sphere is shown in the FIGS. 3 to 5 for thecalibration device 1, i.e. for the coils 18 on the coil card 17, withfour turns of the outer axes and three turns of the inner axes. Theradii in the FIGS. 4 to 6 are normalized to the value of 1.

Furthermore, as the calibration device 1 is very sensitive to a changein temperature, the rotator 2 is placed inside a thermal insulation box(not shown) within which the temperature is controlled by a thermistor,a Peltier element and a controller (not shown) to diminish temperaturegradients. The thermistor is placed close to the rotating coil card 17.The external part of the Peltier element is ventilated by a fan outsidethe field of the magnet via a hose (not shown). Additionally, toestablish a uniform temperature field in the insulation box a ventilatoris built in which stirs the enclosed air, such that a temperaturestability of around ±0.02K is achieved.

In the illustrated embodiment the three cylindrical coils 18 aresituated on the rotating coil card 17 with an orientation substantiallyorthogonal to each other. Each coil 18 has a height of around 5 mm and adiameter of 5 mm and is wound of wire having a thickness of around 0.02mm. The small size of the coils 18 allows the operation in a magneticfield which has to be constant and homogeneous in a finite volume ofminimally 10×10×10 mm³ in size.

Each voltage induced in the coils 18 during rotation in a magnetic fieldis fed to a separate low pass filter and analog to digital converter 20,said analog to digital converter 20 being in form of a 24 bit 4^(th)order Δ-Σ-modulator (delta sigma modulator). The Δ-Σ-modulator convertsits input signal into a stream of bits with a high clock rate or ratheroutput word rate whereas the low pass filter cuts off frequencies higherthen ½ of the output word rate.

Special attention has been paid to the electronics. When positioning thefield sensitive components, such as the coils, the Hall sensors to becalibrated and their current circuits, they are arranged as close aspossible to the rotation center of the cards 17, 17 a. Leads are keptshort, twisted and with small loop surface against parasitic induction.A flexible cable guide 21 contains all the twisted pair leads of thesupply voltages and the digital signals from the analog to digitalconverters. The digital lines can be shared by the Hall sensor card 17 aand coil card 17. Further, the supply voltage leads can be replaced by asystem of collectors, the digital lines can be replaced by a wirelesslink, e.g. infra-red. In that case the flexible cable can be omitted,which reduces vibration.

The effect of induction in the Hall circuit can be determined by makinga calibration with reversed rotation as for example in comparison to therotation shown in the FIGS. 3 to 5.

The stability of the magnetic field during calibration can be monitoredfor example by a fixed NMR-magnetometer (not shown) as close as possibleto the magnet center and by several fixed Hall sensors (not shown).

In the following the calibration of a three dimensional Hall sensorwithin a calibration magnetic field will be described:

-   -   During calibration the cards 17, 17 a rotate continuously to        minimize integration errors around the two orthogonal axes        driven by the shafts 6, 8 such that the full range of the polar        and azimuthal angles is covered.

On the coil card 17 the coil voltages are filtered and the analogfiltered signals are converted into digital signals. The sampled digitalsignals are directed from the coil card 17 to a remote processing unit(not shown), in particular comprised by a microprocessor.

Within the microprocessor, in particular off-line, the filtered coilsignals are reconstructed and integrated to obtain the components of thecalibration magnetic field in angular coordinates relative to the coils,whereas the absolute value of the calibration magnetic field comes from,for example, a NMR magnetometer.

On the Hall sensor card the Hall input voltages as well as Hall outputvoltages are filtered and converted into digital signals. The digitalsignals are forwarded from the Hall sensor card to the remote processingunit or rather micro processor.

Within the microprocessor the digital Hall voltage signals aredecomposed into spherical harmonics on the basis of the angularcoordinates obtained from the coil voltages.

With calibrations done at different absolute values of the magneticcalibration field and temperature, tables of the coefficients ofspherical harmonics for each Hall sensor can be stored for each fieldvalue and temperature value to obtain a calibration table.

Such a calibration table can be used later on to obtain the componentsof a magnetic field with the calibrated Hall sensor on the basis ofmeasured Hall voltages.

The magnetic calibration device of the invention in combination with amathematical algorithm to implement the above described steps allows acalibration of one or more dimensional Hall probes with high precision.The present invention permits the calibration of a 3D Hall sensor whichcan also be built up from three 1D Hall sensors or one 2D plus one 1DHall sensor. In addition, the calibration device of the invention can beused also to calibrate magnetic sensors against an already calibrated 3DHall sensor, with the requirement for a continuous movement beinginapplicable and a shorter calibration time resulting due to the factthat fast movement, i.e. rotation, from one point to another ispossible.

By way of example it is to be mentioned that the present invention canbe used to determine magnetic field strength in synchrotron lightsources or RF power supplies, like beam magnets or wigglers or in thefield of medical physics, like NMR systems or irradiation facilities.

Although modifications and changes maybe suggested by those skilled inthe art, it is the intention of the applicant to embody within thepatent warranted hereon all changes and modifications as reasonably andprobably come within the scope of this contribution to the art. Thefeatures of the present invention which are believed to be novel are setforth in detail in the appended claims. The features disclosed in thedescription, the figures as well as the claims could be essential aloneor in every combination for the realization of the invention in itsdifferent embodiments.

REFERENCE SIGN LIST

-   1 calibration device A rotation direction-   2 rotator B rotation direction-   3 support plate C rotation direction-   4 mounting screw G pole gap-   5 flange J_(Hall) Hall input current-   6 outer shaft V_(Hall-i) Hall input voltage-   7 first worm wheel V_(Hall-o) Hall output voltage-   8 inner shaft V_(Ref.) reference voltage-   9 second worm wheel V_(Reg.) regulation voltage-   10 slide bearing for the outer shaft V_(S) supply voltage-   11 slide bearing for the inner shaft-   12 slide bearing holder-   13 first conical gear-   14 second conical gear-   15 mounting plate-   16 O-ring-   17 coil card-   17 a Hall sensor card-   18 coil-   19 dowel pin hole-   20 low pass filter and analog to digital converter-   20 a low pass filter and analog to digital converter-   21 cable guide-   22 current source-   23 Hall sensor-   24-27 resistor

1. Magnetic calibration device comprising: a mounting means designed tosupport at least one magnetic sensor card being detachably attached andcomprising at least one magnetic sensor, in particular in form of a Hallsensors, to be calibrated and connected to a first analog electroniccircuit with at least one current source as well as at least one firstanalog to digital converter and at least one coil card being detachablyattached and comprising three coils arranged substantially orthogonal toeach other and connected to a second analog electronic circuit with atleast one second analog to digital converters; at least one connectionmeans, in particular in form of a cable or a wireless link, for applyingat least one supply voltage V_(S) to the first and second analogelectronic circuits, respectively, and for guiding digital signals fromthe first and second analog to digital converter respectively, to atleast one processing unit; a magnet for generating a substantiallyhomogeneous and constant calibration magnetic field; and a rotator forrotating said cards in said calibration magnetic field around twosubstantially orthogonal axes.
 2. Magnetic calibration device accordingto claim 1, characterized in that the rotator comprises two conicalgears, such as tooth gears or roll gears, two substantiallyconcentrically arranged shafts and at least one driving unit for theshafts, in particular in form of a stepper engine controller connectedto two engines and/or connected to the shafts via worm wheels. 3.Magnetic calibration device according to claim 2, characterized in thatthe at least one driving unit is arranged outside the calibrationmagnetic field.
 4. Magnetic calibration device according to claim 2,characterized in that the speed and direction of rotation of the twoshafts is controlled by the at least one driving unit to cover the fullrange of polar and azimuthal angles of the unit sphere by a continuousmovement, preferably including a time reversed rotation for compensatinginduction effects in the at least one magnetic sensory.
 5. Magneticcalibration device according to claim 2, characterized in that themagnetic calibration device is at least partly formed by vibrationdamping and nonconducting material, preferably at least one of theshafts comprises heavy, nonconducting material and/or is arranged inslide bearings.
 6. Magnetic calibration device according to claim 2,characterized in that the amount of turns of the inner shaft differsfrom the amount of turns of the outer shaft by one turn within onecalibration cycle, the cable being preferably turned only once withinone calibration cycle.
 7. Magnetic calibration device according to claim1, characterized in that the at least one processing unit is arrangedoutside the calibration magnetic field and stationary.
 8. Magneticcalibration device according to claim 1, characterized in that severalmagnetic sensor cards and/or at least one magnetic sensor card and thecoil card are stackable next to each other, preferably closely spacedapart.
 9. Magnetic calibration device according to claim 1,characterized by at least one dowel pin, screw, plug, clamp and/or clipto precisely and reproducibly position at least one magnetic sensor cardand/or coil card.
 10. Magnetic calibration device according to claim 1,characterized in that one magnetic sensor card carries one 3-dimensionalor one 2-dimensional and one 1-dimensional or three 1-dimensional Hallsensor(s).
 11. Magnetic calibration device according to claim 1,characterized in that the first analog electronic circuit comprises atleast one low pass filter, multiplexer and/or delta-sigma modulator. 12.Magnetic calibration device according to claim 1, characterized in thatthe second analog electronic circuit comprises at least one low passfilter, multiplexer and/or delta-sigma modulator.
 13. Magneticcalibration device according to claim 1, characterized in that the firstand/or second electronic circuit, in particular the at least one lowpass filter, is arranged in the region of the center of rotation of thecards.
 14. Magnetic calibration device according to claim 1,characterized in that the processing unit comprises: means forintegrating the digital coil signals to obtain the components of thecalibration magnetic field in angular coordinates relative to the coils;means for decomposing the digital magnetic sensor output voltages intospherical harmonics on the basis of the obtained angular coordinates;and means for storing the obtained coefficients of the sphericalharmonics as a function of the calibration magnetic field to obtain acalibration table.
 15. Magnetic calibration device according to claim 1,characterized by at least one magnetometer, particular in at least oneNMR magnetometer and/or fixed Hall sensor, arranged within thecalibration magnetic field and connected to the processing unit toprovide the absolute value of the calibration magnetic field to theprocessing unit.
 16. Magnetic calibration device according to claim 1,characterized in that the processing unit comprises: means forperforming a transformation of the obtained angular coordinates foraligning the reference frame with the symmetry axes of the at least onemagnetic sensor.
 17. Magnetic calibration device according to claim 1,characterized by at least one temperature sensing element, comprised bya thermistor or the Hall sensor and connected to the processing unit inorder to supply the temperature within the calibration magnetic field,in particular of the Hall sensor, to the processing unit.
 18. Magneticcalibration device according to claim 17, characterized in that theprocessing unit comprises: means for obtaining the temperature of theHall sensor by decomposing the Hall input voltages depending on themagnitude and direction of the calibration magnetic field and thetemperature into spherical harmonics on the basis of the decompositionof the Hall output voltages.
 19. Magnetic calibration device accordingto claim 1, characterized by a thermal insulating box housing themounting means with the magnetic sensor and coil cards, said thermalinsulating box preferably connected to a control circuit for controllingthe temperature within the box measured in particular by a temperaturesensing element.
 20. Magnetic calibration device according to claim 19,characterized in that the control circuit, preferably comprised by theprocessing unit, comprises a Peltier element for cooling and/or heating,at least one ventilator, preferably driven by an engine outside thecalibration magnetic field, and a controller.
 21. Magnetic calibrationdevice according to claim 1, characterized in that the current source ofthe Hall sensor is either a constant current source or a precise voltagesource with an internal resistance substantially equal to the inputresistance of the Hall sensor.
 22. Magnetic calibration device accordingto claim 1, characterized in that at least two magnetic sensor cards aresupported by the mounting means, one card carrying at least onecalibrated Hall sensor and each remaining card carrying at least oneHall sensor to be calibrated by comparison with the at least onecalibrated Hall sensor.
 23. Magnetic calibration device according toclaim 1, characterized in that the processing unit is comprised by amicroprocessor and/or personal computer.